According to the Campbell's web site, a serving of your soup is 1 cup, which equals 8 oz.
Since you have a 15 oz size, your container has one 8 oz serving and then 7 oz left over (7/8 of an 8 oz serving). So you get one complete serving that matches the nutritional information and then a second serving that's 7/8 of the nutritional information. It's so goofy, because if an 8 oz cup is a serving, why not make the whole thing 16 oz instead of 15 oz??
That's why it's "about two servings" -- because it's almost, but not quite two 8 oz cups.